Recently, the reduction of weight of automobile bodies has increasingly been demanded with the aim of improving the fuel efficiency of automobiles. One of the measures to reduce an automobile body weight is to use a steel material having a high strength. However, as the strength of a steel material increases, the press forming of the steel material becomes increasingly difficult. This is because, generally, as the strength of a steel material increases, the yield stress of the steel material increases and, further, the elongation thereof decreases.
To cope with the above problem, a steel sheet that makes use of strain induced transformation of retained austenite (hereunder referred to as “TRIP steel”), and the like, have been invented to improve elongation and these technologies are disclosed in Japanese Unexamined Patent Publications No. S61-157625 and No. H10-130776, for example.
However, an ordinary TRIP steel sheet inevitably requires a large amount of Si to be contained, as a result the performance of chemical conversion treatment and hot-dip galvanization on the surface of the steel sheet deteriorates and, therefore, the members to which the steel sheet is applicable are limited. In addition, in a retained austenite steel, a large amount of C must be added in order to secure a high strength and, as a result, problems of welding, such as nugget cracks, arise.
With regard to the performance of chemical conversion treatment and hot-dip galvanization on the surface of a steel sheet, inventions that aim to reduce the Si amount in a retained austenite TRIP steel are disclosed in Japanese Unexamined Patent Publications No. H5-247586 and No. 2000-345288. However, through the inventions, though an improvement of the performance of chemical conversion treatment and hot-dip galvanization, as well as ductility, can be expected, an improvement in the aforementioned weldability cannot be expected. Moreover, in the case of a TRIP steel of 980 MPa or more in tensile strength, the yield stress is very high and, therefore, the problem has been that the shape freezing property of the steel deteriorates at the time of pressing or the like. Further, in the case of a high strength steel sheet of 980 MPa or more in tensile strength, the occurrence of delayed fracture is a concern. Another problem is that, as a TRIP steel sheet contains a large amount of retained austenite, voids and dislocations are formed, in quantity, at the interface between a martensite phase formed by strain induced transformation and other phases in the vicinity of the martensite phase, hydrogen accumulates the interface and, then, delayed fracture occurs.
Further, as a technology of reducing a yield stress, a dual phase steel (hereunder referred to as “DP steel”) containing ferrite has so far been known as disclosed in Japanese Unexamined Patent Publication No. S57-155329. However, the technology requires that a cooling rate after recrystallization annealing is 30° C./sec. or more and the cooling rate is insufficiently achieved in an ordinary hot-dip galvanizing line. Furthermore, the target tensile strength of the steel sheet is 100 kg/mm2 at the highest and therefore a high strength steel sheet having sufficient formability has not always been realized.